In order to join a tube made of plastic, metal, or rubber to another tube, to a pipe fitting, or to a plumbing fixture, it is frequently necessary to provide the tube end with a special fitting that can mate. For instance one side can have a tubular outwardly tapered extension formed with sawtooth-section teeth so that it can be jammed into a rubber hose, or it can have an outwardly open annular groove of outwardly flaring radial dimension so that a plastic hose can be fitted to it and clamped in place.
On the side attached to the tube end the fitting typically has a seat into which the tube end, which may be provided with a radially outwardly projecting retaining formation either formed directly on it in a metal tube or constituted by a separate ferrule with a plastic tube, can fit. Typically the inner end of the seat is provided with a seal ring that engages snugly between the fitting and the tube end to form the necessary leak-free connection.
A clip mechanism is provided to retain the tube end in the seat, normally coacting with the retaining formation on the tube end. Such a mechanism operates automatically to secure the tube end in place when it is pushed home in the fitting. Normally the tube end, once locked in place, cannot be removed from the fitting and the fitting cannot be reused.
Another problem with the known fittings is that it is impossible to determine if in fact the tube end is fully engaged and retained in the fitting. The retaining clip may not have actually been engaged behind the retaining formation, but the tube end might be pushed far enough into the fitting that they are seemingly solidly connected together. Only when pressurized will the two parts separate, sometimes after some use. When used in a brake system, the result can be depressurization of the system and failure of the brakes.